1. Field of the Invention
The present invention relates to an electric motor control device which performs drive control for an AC electric motor.
2. Description of the Background Art
Conventionally, electric vehicles are known that use, as a driving force source, an AC electric motor which performs power running operation to generate a travel driving torque during traveling and which performs regenerative operation to generate a regenerative braking torque during braking operation.
In an electric vehicle drive system, a DC power supply implemented by a secondary battery such as a lithium ion battery is connected to an inverter composed of a capacitor and a plurality of semiconductor switches, and to this inverter, an AC electric motor is connected as a load. The inverter turns on and off the plurality of semiconductor switches at a predetermined switching frequency, to convert DC power of the DC power supply into predetermined AC power, thereby adjusting the torque or rotation rate of the AC electric motor which is a load. As the AC electric motor to be used in an electric vehicle, a permanent magnet synchronous electric motor which is efficient is often used. Depending on the operation state, the AC electric motor operates as an electric generator and charges the DC power supply with regenerative power generated through the power generation.
The operation principle of the inverter is well known and is not explained here.
In the electric vehicle drive system, in order to protect the battery being the DC power supply from overvoltage and overcurrent, a switch section which disconnects the battery from the inverter as necessary is provided. Examples of opening conditions for the switch section include that the voltage of the battery has become greater than or equal to a predetermined value during regenerative operation of the electric motor, that the voltage of the battery has become less than or equal to a predetermined value due to consumption of the battery, and that current flowing in the battery has become greater than or equal to a predetermined value.
In such a system, there are cases where the switch section is opened during regenerative operation of the electric motor, to be disconnected from the DC power supply. Moreover, even in the case of a system not including the switch section, there are cases where the DC power supply is disconnected from the inverter due to breakage of the power line between the DC power supply and the inverter. In such cases, the regenerative power flowing from the electric motor into the inverter cannot be charged to the battery, and instead, is charged to the capacitor in the inverter. This may cause overvoltage in the capacitor, resulting in damage of the capacitor.
As a countermeasure to this, there is a technique in which when the inverter is disconnected from the DC power supply, all semiconductor switches in the inverter are turned off to stop operation of the inverter, thereby stopping regenerative operation of the AC electric motor. However, with this technique, the regenerative braking torque of the electric motor is suddenly reduced, and the regenerative braking of the vehicle is suddenly disabled. This significantly lowers the operability for the driver. In addition, when the operation of the inverter is suddenly stopped, an excessive surge current may flow into the inverter due to induced voltage of the electric motor. This may damage the semiconductor switches and the like.
As another countermeasure, there is a technique of additionally providing a discharge circuit in which regenerative power flowing from the electric motor into the inverter is consumed through heat generation, whereby excessive regenerative power flowing into the capacitor is consumed in the discharge circuit. However, with this technique, since the discharge circuit is additionally provided, the size of the inverter is increased. In particular, if a large amount of regenerative power is to be consumed in the discharge circuit, the discharge circuit needs to be configured, using an element having a large withstand power. This may become an obstacle in realization of downsizing and low cost of the inverter. For an inverter of electric vehicle that needs to be disposed in limited space in the vehicle, the presence of the obstacle in downsizing thereof poses a significant problem.
This problem can be addressed by a technique of reducing the regenerative power to be flowed to the DC power supply side. As the technique, Japanese Laid-Open Patent Publication No. 9-121561 discloses a method for processing regenerative power for the inverter characterized in that: the switching frequency of the inverter is controlled so as to be changed in accordance with variation in DC voltage due to regenerative power; and the regenerative power is consumed by switching loss in switching elements of the inverter.
With this technique, if regenerative power is increased, the switching frequency of the inverter is increased in accordance with the increase thereof, to increase the switching loss. Then, the regenerative power is consumed through the switching loss, thereby realizing a small inverter in which a regenerative power consuming component such as a discharge circuit can be omitted.
However, with the approach disclosed in Japanese Laid-Open Patent Publication No. 9-121561, when regenerative power has been increased, the switching frequency is increased with only the switching loss taken into consideration, and a property that the loss in the electric motor is dependent on the switching frequency is not taken into consideration. Accordingly, the regenerative power cannot be effectively consumed through losses in the inverter and the electric motor.
Specifically, in a switching frequency range often employed in an inverter for electric vehicle, it is general that, if the switching frequency is increased, the inverter efficiency is reduced and the electric motor efficiency is increased as shown in FIG. 3. The overall efficiency in total is not necessarily reduced even if the switching frequency is increased. That is, there are cases where the overall loss is decreased when the switching frequency is increased. In such a case, if the switching frequency is increased such that the switching loss is increased, regenerative power to be consumed in the inverter and the electric motor will be decreased. As a result, regenerative power to be flowed to the DC power supply side is increased. Thus, this technique poses a problem that downsizing of the inverter cannot be realized.